This invention relates to a fuser system that includes a closed loop control that controls a fuser's nip pressure.
In the art of xerography or other similar image reproducing arts, a latent electrostatic image is formed on a charge-retentive surface, i.e., a photoconductor or photoreceptor. To form an image on the charge-retentive surface, the surface is first provided with a uniform charge after which it is exposed to a light or other appropriate image of an original document to be reproduced. The latent electrostatic image thus formed is subsequently rendered visible by applying any one of numerous toners specifically designed for this purpose.
It should be understood that for the purposes of the present invention, the latent electrostatic image may be formed by means other than by the exposure of an electrostatically charged photosensitive member to a light image of an original document. For example, the latent electrostatic image may be generated from information electronically stored or generated, and this information in digital form may be converted to alphanumeric images by image generation electronics and optics. The particular method by which the image is formed is not critical to the present invention, and any such suitable method may be used.
In a typical xerographic device, the toner image formed is transferred to an image receiving substrate, such as paper. After transfer to the image receiving substrate, the image is made to adhere to the substrate using a fuser apparatus. To date, the use of simultaneous heat and contact pressure for fusing toner images has been the most widely accepted commercially, the most common being systems that utilize a pair of pressure engaged rolls.
The use of pressure engaged rolls for fixing toner images is well known in the art. See, for example, U.S. Pat. Nos. 6,289,587, 5,998,761, 4,042,804 and 3,934,113.
At the time of initial set-up of a xerographic device, the fuser system is set to be within certain specifications for, e.g., dwell time (nip width/process speed), paper velocity and creep. Dwell time is one of the more significant drivers of image fix and quality. Paper velocity is an important factor in paper handling. Creep, which is the release surface's extension in the nip, is important with respect to enabling self-stripping of the paper from the fuser roll. These specifications are set by, for example, setting a roll rotation speed for the paper velocity and setting the nip width for the dwell time and creep.
Once initially set, the nip width and nip uniformity of a typical fuser is not changed during operation of the xerographic device. Unfortunately, several internal and external factors can cause the fuser system to drift outside of the designated specifications. For example, in a typical soft-on-hard roll pair in which the soft roll is the driving roll, the fuser system may begin operating outside of specifications due to, e.g., hardening of the roll materials over time. Typical fuser roll systems include some materials such as silicone materials that tend to become harder over time at unpredictable rates. This hardening causes large reductions in both dwell time and creep, which causes premature failure (e.g., smaller nip widths that lead to insufficient fixing of the toner image and/or poor image quality, as well as to poor stripping of the image receiving substrate).
In addition to these failure modes, it is at times desired that the nip width and nip uniformity in a fuser be altered on demand. For instance, the fusing quality on thick paper is improved with large nip widths, and the fusing quality on thin papers is often improved with small nip widths. The fusing latitude in the presence of varied media and images, therefore, is improved if the nip width can be accurately set and controlled.
Typically, resetting the nip width to improve fusing latitude or to compensate for system failures due to the fuser system falling out of specifications has been dealt with by either (a) having a technician re-set the nip on site and/or (b) setting the nip width far above specifications at the factory, permitting the device to operate longer before falling out of specification. However, each of these ‘solutions’ has serious problems. Using technicians to reset the nip requires an on site visit by a technician and down time of the device. Initially setting the nip width high above specifications usually causes paper handling and stripping issues, especially with lightweight papers.
Maintaining nip width uniformity is as critical as maintaining the average nip width, as a nip width uniformity out of specification results in two major failure modes. The first is that the axial variation of nip width and pressure results in axial variation of toner adhesion/fix and axial variation of toner gloss, which can cause the fuser to fail to meet print-quality requirements. The second is that the axial nip uniformity also controls the fuser's paper handling and wrinkling performance, so variations in uniformity can cause the fuser to fail for wrinkling, mis-stripping, or other paper-handling reasons.
What is required is an improved inline method where the machine itself measures and adjusts the nip width and nip uniformity to maintain the fuser system within operational specifications.